LED assemblies typically include a light emitting diode paired with a layer or layers of material that can convert the emission of photons from the light emitting diode to alternate wavelengths of energy. Conventional LED assemblies used for consumer lighting (white light) are composed of two primary components. The two components include the light emitting diode (semiconductor junction) and a layer of a converter material. The layer of converter material is a component that, when light is passed through it, will convert the incoming light to light of a different wavelength. The light converting material is configured so that it is in line with the photo-emission of the LED and serves the purpose of down converting (or in rare instances, up-converting) a portion or all of the photo-emission of the LED to a select region of the electromagnetic spectrum, typically with the goal of producing light perceived as white light by the eye. Down converting involves converting a higher-energy input to a smaller-energy output with longer wavelengths, and up-converting involves converting a smaller-energy input to a higher-energy output with shorter wavelengths, with multiple photons being combined and re-emitted as a single photon.
The light emitting diode used in a most classic consumer configuration emits in the near-UV part of the electromagnetic spectrum with a monochromatic photo emission peak ranging from approximately 390-440 nm with a full width half max of the emission peak being typically on the order of several tens of nanometers. Thus, the diode emits over a very narrow emission range. There are a variety of LED types used to produce this near-UV emission including, for example, but not limited to, InGaN, GaN, GaAsP, GaP, etc.
A converting material layer is typically mounted or adhered onto or in direct line with an LED or path of the photons emitted from an LED. The converting material layer is typically uniform. The short-wavelength photons (typically near UV) from the LED pass through the converter material layer, and a substantial portion of the photons are absorbed by the converter material and optically active atoms within the converter material, and then are re-emitted as a lower energy wavelength of light. In some rare instances the converted light may be of higher energy wavelengths than the incident light, although this is not common. In most instances, intentionally, a small percentage of the LED photons are allowed to pass through the converter material layer without being converted, adding to the net emission spectrum of the LED assembly because the particular wavelength of light emitted by the diode may be desirable.
The most common converter material used for white light production is a cerium doped yttria aluminum garnet ceramic (Ce:YAG) either in the form of a small plate (converter plate) in the form of a monolithic layer, or as a powder suspended in a glass or polymer matrix, known as phosphor in glass (PIG). The glass matrix is usually a low-temperature glass so that the phosphors are not altered by the heat, because the phosphors may be irreversibly damaged if exposed to temperatures that are too high. The powder suspension and the converter plate function in the same manner, and there may be an additional plastic layer through which the light passes after the converter plate. The configuration of a near-UV LED paired with the Ce:YAG converter material produces a net output of light that is perceived by the human eye as white light and is the classic method of producing white-light LEDs assemblies. This method offers very high luminous efficacy.
Despite the benefit of very high efficiency, the prior described technology can lack the ability to produce a full spectrum or complete spectrum “white” light and can exhibit poor color rendering. The emission spectrum of the UV LED paired with a Ce:YAG converter material, when resolved into its spectral components, is primarily di-chromatic, i.e., possessing only two peaks. One spectral peak is centered around the emission wavelength of the base LED (blue or near ultra violet) and the second spectral peak is centered around the primary emission wavelength of the converter material (approximately, but not exactly yellow/green, 560 nm, for Ce:YAG). Converter materials other than Ce:YAG can be used to produce a wide range of colors besides white. For example, Ce:LuAG converter plates can be used to produce a bright green color when paired with a near UV LED.
Manufacturers of white light LED assemblies have attempted to shift the emission spectrum to be warmer (containing more red) and/or be more complete, i.e., achieving a continuous full spectrum white light. The current methods used by manufacturers to produce a more complete white light spectrum include incorporating additional luminescent materials (phosphors), either as part of the converter material layer or in addition to it, such as in or on the plastic or glass casing around the LED assembly. For example, the plastic housing around an LED light bulb may contain additional phosphors (in addition to the Ce:YAG converter material) which are selected and utilized in careful proportions to add specific parts of the visible spectrum i.e. green, yellow, orange, etc.
The PIG (phosphor in glass) configuration relies on a mixture or blend of phosphors, suspended in a sufficient transparent medium, placed in front of the near UV LED, to produce a fuller spectrum of white light, or any desired color. In the PIG configuration there is an obstructed path of photons through the PIG material as there is no organized structure and some phosphor grains may obstruct the path of photons, resulting in loss of potential number of photons emitted. As the phosphor grains are randomly distributed in the host matrix, there is a resulting loss of efficiency in production of light, albeit color rendering achieved may be much improved compared to the simple Ce:YAG converter plate inclusive LED assembly.
Quantum dot converter plates us a technology similar to a laser jet printer to print phosphors onto a substrate of glass or acrylic. Alternatively, printing can be done directly onto a diode junction. Another technique involves encapsulating a wide range of phosphors into a layer, with the distribution not being finely tuned.